rustdoc: Better sorting criteria for searching.

rustdoc: Better sorting criteria for searching.

This essentially rewrites the sorting algorithm, which relied on
the implementation-defined handling of non-consistent sorting function
(cf. ECMA-262 5th edition, section 15.4.4.11)
and was also a bit inefficient.

The new criteria expands the prior criteria while adding these ones:

- The current crate is always preferred over other crates.
  (Closes #13178)
- An item with a description is preferred over one without it,
  if item names match. This is a heuristic assuming that
  the documented item is more likely to be relevant.
- An item with no literal occurrence of search query is handled correctly.